Interaction between microtubules and membranes is essential to many cell functions yet, in contrast to cytoskeletal actin, nothing is known about how it occurs. We chose a Trypanosomatid protozoan because its skeletal microtubules are found in close opposition to the plasma membrane, and electron microscopy had revealed periodic crosslinks between tubules and membrane (as well as between adjacent microtubules). We began by studying 3 proteins that appeared prominent in isolated cytoskeleton. Antibodies to one of them stained glycosomes in cell sections, and the protein proved to be a glycolytic enzyme which adhered to microtubules when released by homogenization. The other 2, detergent-soluble proteins, are absent from glycosomes. One of them cross-links microtubules in vitro. Both bind to soluble tubulin or microtubules with -log KD = 7 (one prefers tubulin, -log KD =8). Binding curves show positive cooperativity with maximum of 0.5 to 1.0 mol bound per mol tubulin. However, we have not yet established whether either protein is part of the cellular crosslinks.